Automatic focus with variable magnification

ABSTRACT

A system for automatically focusing images of different magnifications to be reproduced onto a light sensitive medium. First and second optical patterns of a wavelength to which the light sensitive medium is not responsive are projected from first and second displaced light pattern producing means to the projection plane of the images to be reproduced and back to the light pattern producing means. The light pattern producing means have transparent portions therein such that when the returned optical patterns are defocused portions of the defocused optical patterns are transmitted through the transparent portions. The transmitted light is then mechanically modulated and then converted to an electrical signal which is amplified, filtered and then demodulated to provide a signal having the polarity needed to drive a servo system such that focus of the images to be reproduced is achieved.

BACKGROUND OF THE INVENTION

In microfiche blowback systems it is highly desirable to use amagnification that can be varied readily. This is because various formsof microfiche use different reduction ratios, and also because specificusers of the system have various preferences in the blowbackmagnifications that are utilized. The magnification of the systemadheres to the fundamental relationship ##EQU1## WHERE D₂ IS THEIMAGE-TO-LENS LENGTH (THE LONG CONJUGATE LENGTH) AND D₁ IS THEOBJECT-TO-LENS LENGTH (THE SHORT CONJUGATE LENGTH). The lens equation##EQU2## RELATES FOCUS TO THE LONG AND SHORT CONJUGATE LENGTHS. Insystems where the focal length (f) of the lens is fixed, magnificationis often achieved by changing the long conjugate length d₂. Fromequation (2) it is seen that changes in the long conjugate length willaffect focus and that to maintain focus such a change will require acorresponding change in the short conjugate length.

Generally, magnification control is achieved by manual control of one ofthe path lengths d₁ or d₂ with automatic control of the second pathlength being maintained. In this type of magnification control of theprior art, the second path length is moved in strict accordance with thelens equation (1), it being assumed in those systems that focus will beachieved if the relationships of the lens equation (1) are maintained. Adisadvantage of this type of system is, for example, a lack ofcompensation for variations, such as temperature expansion orcontraction of the support structure for the projection lens or theobject. Since at high magnification and low f/numbers, tolerances may bevery small between the lens and the object plane, on the order of 0.001inches, systems which rely solely on the solution of lens equation (1)are not always satisfactory.

In automatic focus systems, such as described in the U.S. Pat. Nos.2,968,994 and 3,421,815, means are provided to evaluate the focus of anactual image and to control the movement of some element of the opticalsystem to maintain a focus condition once it has been achievedregardless of extraneous movement. These systems, known as optical probefocus servo systems, utilize a light source which projects a targetimage of high resolution backward through the optical system to theobject plane, with the target image reflected from the object beingprojected back through the image system and reimaged at the targetplane. If the long and short conjugate lengths are correct in relationto the optical probe imaging lens focal length, the return image will besuperimposed on the target and no focus adjustment need be made. If theconjugate lengths are incorrect, that is, not in accordance with lensequation (1), the condition occurs in which the returned target image isin a defocused condition at the target plane. This condition can besensed, and used to generate a servo-control signal to effectrepositioning of the image plane.

Several problems relate to the auto focus system described in thereferenced patents. One problem is that the system only maintains focusafter it has been manually set, and that it must be manually set eachtime magnification is changed. A second problem involves the focuscontrol system, which varies the optical path length between lens andobject by direct movement of the object plane. Within the tolerances ofthe system, this direct movement may be difficult to control. A thirdproblem which exists is stray light, there being no means todifferentiate between light arriving at the sensor as a result of afocusing condition and light that arrives at the sensor due toextraneous sources of light such as light which will be scattered in theoptical system.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to provide an improved systemfor achieving automatic focus with variable magnification.

It is a further object of the invention to provide an improved systemfor achieving automatic focus with variable magnification which issubstantially insensitive to extraneous motion.

It is still a further object of the present invention to provide animproved system for achieving automatic focus with variablemagnification which is insensitive to stray light.

SUMMARY OF THE INVENTION

In accordance with the invention, the optical path used for focussensing is essentially the optical path used for imaging. When adefocused condition is sensed, which may be due to a change in theoptical path length of the long conjugate in order to achieve adifferent magnification, the optical path length of the short conjugateis automatically changed. Changes in the optical path length of theshort conjugate are achieved by movement of optical wedges which arelocated between the projection lens and the object to provide very fineand accurate control of the projection lens-to-object distance.Additional advantages are achieved by using a projection lens which istelecentric.

The focused detection system utilizes two mirror grids with alternateclear (light transparent) and light reflecting portions and a rotatingoptical modulator which is comprised of spaced opaque and transparentbars of onehalf the width of the clear and reflecting portions of thegrids. In a slightly defocused system condition, an image of the gridpattern will have a light distribution pattern such that light will passthrough the edges of the clear portions of the mirrored grids. Themodulator scans across the clear portions and alternately blocks lightat the edges of the clear portions and then light at the centralsections of the clear portions. This will produce a modulation of thedefocused light that is passing through the clear portions. The mirroredgrids are displaced relative to the modulator such that the modulationsignals produced by each mirrored grid in association with the modulatordisk will be 180° out of phase with each other. The composite signalwill carry a modulation which has a phase relationship corresponding tothe channel which is furthest from focus. The composite signal isdemodulated to provide a direct current control signal which is used todrive a motor which in turn drives one of the optical wedges in adirection toward focus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the automatic focus withvariable magnification system.

FIGS. 2, 3 and 4 are illustrative plane views of components of thesystem of FIG. 1.

FIG. 5 is a schematic representation of the interaction betweencomponents of the system of FIG. 1.

FIGS. 6A and 6B, and 7A, 7B and 7C are waveforms appearing at designatedpoints of the system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a system for projecting an imagelocated at an object plane, such as one of the microimages on microfiche2, onto a light responsive image plane, which may be the surface of aphotoreceptor 4. The microfiche is supported by a clear platen 6 whichhas on regions of its top surface a light reflecting coating 7 in theform of a grid, as shown in FIG. 2. In lieu of coating 7, a dichroicmirror may be provided over the entire platen glass. The dichroic mirrorpasses wavelengths used for projecting the microfiche image but reflectswavelengths used for the auto focus probe.

During reproduction, the microimages of microfiche 2 are positioned overthe apertures 8 in the coating 7 such that light from a source 10 can beprojected through one of the microimages to the photoreceptor 4 via theoptical path including the optical wedges 12, the projection lens 14,the mirrors 16 and 17, the trombone mirrors 60 and 61, and the beamsplitter 18. By a mechanical arrangement well known in the prior art,one of the trombone mirrors is moved, as shown graphically by the doubleheaded arrow labeled magnification control, to change the distancebetween the lens 14 and the surface of photoreceptor 4, that is, tochange the long conjugate length of the optical path, to thereby changethe magnification of the optical projection system in accordance withthe reduction rate of the microimages of microfiche 2. The light fromthe source 10 is of a wavelength to which the photoreceptor 4 isresponsive, for example, blue light.

As noted, changing the long conjugate length without changing the shortconjugate length will result in a defocused image at the photoreceptor4. Focus control is achieved by movement of one of the optical wedgeswhich movement changes the short conjugate length. Movement of theoptical wedge is controlled by a signal provided by spaced, identicalmirrored grids 20 and 21 and adjacent, rotating optical modulator 22which is driven by a synchronous motor 23. The mirrored grids arepreferably disc-shaped and each is comprised of a plurality of lightreflective surfaces 24 supported by a clear glass base 25, such thatlight transparent portions 26 exist between light reflective surfaces24, as shown in FIG. 3 which is a plane view of a portion of one of thegrids. Surfaces 24 are of equal size and preferably in the form oftruncated sections of a circle, as shown, with each portion 26 occupyingan angular segment of base 25 equal to that angular segment occupied byeach surface 24, i.e., θ₁ = θ₂. The light modulator 22 is preferablydisc-shaped and consists of a grid of alternate opaque and lighttransparent pi-shaped segments 89 and 90, respectively, of equal size,which segments 89 and 90 have twice the spatial frequency (one-half thewidth) of the reflective surfaces 24 of the mirrored grids. FIG. 4 is aplane view of modulator 22 with only a small percentage of the alternateopaque and transparent segments 89 and 90 shown. The mirrored grid 20 ispositioned in relation to the modulator 22 such that is occupies a phaserelationship with the modulator segments which is 180° different fromthe phase relationship that the mirrored grid 21 has with the modulatorsegments. That is, referring to FIG. 5 which is a view of the grids 20and 21 looking back through the modulator 22 with the surfaces 24 andthe segments 89 and 90 shown as rectangles for ease of illustration, theopaque segments 89 of the modulator 22 obstruct the middle portions ofthe reflective surfaces 24 of grid 21 when they obstruct only the edgeportions of the reflective surfaces 24 of grid 20. This phaserelationship will provide, following demodulation, an output signal ofone polarity from channel one (the channel including grid 20) and asignal of the opposite polarity from the other or second channel (thechannel including grid 21). The composite output signal will indicatethe direction in which one of the wedges 12 must be moved to achievefocus.

Each of the mirrored grids 20 and 21 is illuminated via a condenser lens31 and a beam splitter 32 by light from a source 30 which has awavelength to which the photoreceptor 4 is not responsive, for example,red light. The mirrored grids form the object or target which isprojected by the reflective surfaces 24 back to the reflective coating 7on the surface of the platen 6. For grid 20, the projection pathincludes mirror 40, beam splitter mirror 18, trombone mirrors 60 and 61,mirrors 17 and 16, lens 14 and wedges 12, whereas the projection pathfor grid 21 includes the same elements of the projection path for grid20 except that it utilizes mirror 41 in lieu of mirror 40. The mirroredgrids 20 and 21 are equidistant from a point A which point is the samedistance from beam splitter 18 as the surface of photoreceptor 4 is frombeam splitter 18. This placement of the grids provides for measurementof the focus condition on either side of the plane of best focus.

Situated on the side of the rotating modulator 22 remote from the grids20 and 21 is a photodetector 50 which is responsive to light from source30 and which, due to the orientation of the mirrors 40 and 41, receiveslight from both channels 1 and 2. The spectral filter 51 blocks lightnot of the wave length of the light from source 30.

The output signal of the detector 50 is amplified by an amplifier 52 andthe amplified signal is supplied to a band pass filter 54 which is tunedto the modulation frequency, that is, for example, filter 54 is tuned to6 KC when modulator disk 22 is rotated at 60 revolutions per second andhas 100 interspersed opaque and transparent sections. The filteredsignal is supplied to a demodulator 56 which has a reference signalwhich is generated when the opaque sections of the rotating modulator 22block the flow of light from light emitting diode 80 to photodiode 81.In the example specified, the reference signal is a 6 KC signal. Theoutput of the demodulator 56 is supplied to a filter 58 to remove noise,specifically, for the example given, 6 KC modulation noise, with theoutput of the filter 58 supplied to the control circuit 59.

The output of the amplifier 52 is also supplied to a low pass filter 62whose band pass is a function of the speed at which the focus controlsystem can drive through focus, which, for the example given, would beless than 10 hertz. The output of the low pass filter 62 is supplied toa conventional peak detector-comparator circuit 64, with the output ofthe peak detector-comparator circuit 64 being supplied as an input to alogic circuit 66 which also receives a focus search command input signalof the type to be described later. The output of the control logiccircuit 66 is supplied as a control signal to control circuit 59.Control circuit 59 and logic circuit 66 are conventional circuits andmay be a conventional comparitor and electronic switch circuit and aconventional comparitor and flip flop, respectively. The output of thecontrol circuit 59 is amplfied by amplifier 68 and the amplified signalis supplied to a motor 70 which is coupled to one of the wedges to movethe wedge to the right or to the left, as shown by the double headedarrow, to achieve variation of the short conjugate length and hencefocus.

In operation, the mirrored grids 20 and 21, illuminated from the lightsource 30, form the object or target which is projected through theoptical system. Images of the reflective surfaces 24 of grids 20 and 21are formed by the projection lens 14 at the reflecting surface 7 of theplaten 6. This light is then reflected back through the projection lens14 and mirror system to be reimaged at the source, that is, the mirrorgrids 20 and 21. If this second image is in focus at the mirrored grids,all reimaged light will be reflected back to the illuminating source 30.If the image is out of focus at the mirrored grids 20 and 21, portionsof the light will pass through the light transparent portions 26 ofthese grids. As noted, the modulator has alternate transparent andopaque portions which opaque portions occupy 50% of the area of lighttransparent portions 26. Thus, if a uniform illumination passes throughthe light transparent portions 26, the transmitted light intensity doesnot vary as the modulator 22 scans across the light passing throughlight transparent portions 26. As previously stated, an image of themirror grids 20 and 21 which is in focus will fall superimposed with themirror grids and no reimaged light will pass through the portions 26.However, in a slightly defocused condition, the reflected image willhave a light distribution pattern such that some reimaged light willpass through the portions 26 at the edges thereof as shown in FIG. 3.Referring to FIG. 5, it can be seen that as the modulator 22 scansacross the portions 26 (the portions between surfaces 24) the opaqueportions 89 of the modulator 22 will alternately block the edges of theportions 26 and then the central portion of the portions 26. In effect,it will produce a modulation of the defocused light that is passingthrough the portions 26. In addition, the two sensor channels, i.e.,grids 20 and 21, have a reflective surface and modulator positionalrelationship such that when portions 26 of one grid are being blocked bythe modulator grids at its edges, the other set of portions 26 of theother grid are being blocked by the modulator grids in the central areathereof. Thus, the defocused light is modulated such that themodulations will be 180° out of phase with each other. If these lightlevels are of equal amplitude, recombining the two light beams willproduce a resultant light beam which is not modulated. This conditionimplies that the images of both grids 20 and 21 are equally out of focus-- a condition which can only exist when the plane of best focus fallsat a point which is halfway between the mirrored grids, 20 and 21, thatis, at point A. A shift of this focus position one way or the other(either due to an adjustment of the magnification control or due touncontrolled movement of a system component) will cause the defocusedcondition at one grid to be reduced and the defocused condition at theother grid to be increased. Under this condition, the modulated level ofthe light passing through the portions 26 of the other grid willincrease. The sum of these two light beams will thus carry a modulationwhich has a phase relationship corresponding to the channel which isfurthest from focus.

This modulated light falls on detector 50 and generates a proportionalsignal. This signal is amplified by bandpass amplifier 52 whichamplifies at the modulation frequency. The amplified signal is thensynchronously demodulated using the reference signal which is generatedby light emitting diode 80 whose light is modulated by the modulatordisk 22 and then detected by detector 81. The demodulated signal isfurther filtered to remove modulation noise and amplified to provide apositive DC signal when the plane of best focus falls closest to onemirrored grid and a negative DC signal when the plane of best focusfalls closest to the other mirrored grid. This DC control signal is usedto drive motor 70 which in turn controls the movement of one of theoptical wedges 12 across the projection axis. The optical wedges 12 inturn adjust the optical path length between the projection lens 14 andthe platen reflective surface 7 causing the plane of best focus to shiftback to point "A" located half-way between the two mirrored grids 20 and21. At this point, signal modulation will be zero, and the motor 70 nolonger drives.

In order to best understand the operation of the automatic focus systemcontrol circuits, it is best to examine the distribution of light whichis reimaged at one of the mirrored grids. This is best done bydescribing what one would view through a portion 26 of one of the grids,in terms of light intensity and distribution passing through thatportion, as the system is focused from one extreme of defocus, throughfocus, and into the other extreme of defocus. Starting at an extremedefocus condition, the light passing through the portion 26 will have arelatively low intensity. This is due to the fact that image blurr is soextreme that the returned light, the light reflected from surface 7, isdistributed over a large area, several times larger than the mirroredgrid size. As focus is approached, the area covered by the returnedlight decreases resulting in an increase of energy which passes throughthe portion 26. As focus is approached, the total light passing throughthe portion 26 reaches a peak at which time the light distribution willstart to change, getting darker in the central section of the portion 26and brighter at the edges. Continuing toward focus, this lightredistribution will become more apparent, with the total light energydecreasing. When focus is reached, the light passing through the portion26 is confined to the immediate vacinity of the edge (with the exceptionof a uniform, unchanging background light caused by scattering). In thefocused condition, the total light passing through the portion 26 hasreached a minimum. Passing through focus toward the opposite extremedefocus condition, the change in light level and light distributionpasses through the phases described above, but in the reverse order.

As previously explained, the light modulator 22 consists of a grid ofopaque and transparent bars, which have twice the spatial frequency(one-half width) as the portions 26 of the mirrored grids. As themodulator scans across the grid portions 26, light which is distributednear the edges of the portions 26 is modulated, whereas uniform light isnot modulated. If the light passing through the portions 26, and throughthe modulator 22 is detected, and the system is scanned through focus asdescribed above, the single channel detector output signal will appearas shown in FIG. 6A, which, for the example given, has a 6 KCmodulation. The output of bandpass amplifier 54, tuned to the modulationfrequency, is shown in FIG. 6B. The dc output of the demodulator 56,after filtering, will have a form, which corresponds to the positive ornegative envelope shown in FIG. 6B, the polarity being dependent on thephase of the modulation with respect to the phase of the demodulatorreference signal.

In the above description only one of the mirrored grids, and itsresultant signal, have been considered. The second channel operates inan identical manner, with the exception that the focused condition isoffset (due to a different optical path length) and the modulation phaseis 180° different, resulting in the opposite polarity output. If theoutput of the demodulator is first plotted with only one optical channeloperating, then plotted with only the other channel operating, the twoplots will appear as shown in FIG. 7A. When both channels are operating,the output of the demodulator will appear as shown in FIG. 7B. Referringto FIG. 7B, it can be seen that there are three intervals of the signalas the system is moved through a wide range of focus. The proper focuscondition is located at the central crossover point "F". The focuscontrol motor 70 is therefore connected such that a positive output fromthe demodulator 56 will drive the focus toward the left (referring toFIG. 7B) while a negative signal would drive the focus toward the right.It can thus be seen from FIG. 7B, that if the system has been presetwithin the "lock in" range, the system will automatically focus to thebest focus condition; however, if the system has been preset to a pointoutside the "lock in" range, the system will drive away from the pointof best focus. Accordingly, the system must determine when it is focusedwithin the "lock in" range. This determination is made by means of thecombination of the filter 62, peak detector-comparator 64, and logiccircuit 66. As previously mentioned, the total light transmissionthrough the portions 26 of the grids 20 and 21 reaches a peak on eitherside of focus. This condition provides a means to locate the "lock in"range, as shown in FIG. 7C which is the unprocessed signal from detector50 with both channels operative. As can be seen, the amplitude of thesignal reaches a peak outside the "lock in" range, and passes through aminimum at the focus null.

The signal 7C is processed to determine the focus "lock in" range. Whenthe system is in a standby condition, such as when changing microfiche,the focus control is driven to one extreme position (by means notshown). Initiation of the operate cycle supplies the focus searchcommand signal to control logic 66 and causes the focus control to scantoward the opposite extreme focus condition. The detector circuit 64,monitoring the signal of FIG. 7C, generates a "lock in" control signalwhen the signal passes through a peak, such as peak B of the signal of7C. In the meantime, a zero crossing detector 80 monitors thedemodulated signal 7B, i.e., the output of demodulator 56, and generatesa logic signal when the zero crossing point C is reached. These logicsignals, in turn, control circuit 59 such that the signal of 78 ispassed by the control circuit 69 only when the signal of FIG. 7B has avalue between zero crossing points C and C'. Once within the "lock in"range, the system maintains focus by virtue of the closed loop servocontrol.

An important feature in the proposed system utilizes the optical wedges12 which provide the means to vary one of the optical path lengths.Since it is necessary to provide a very fine and accurate control of thelens-to-object distance, any mechanism which moves the lens or filmplane must be smooth operating, free of backlash, and also free ofmovement which may be orthogonal to the optical axis. The optical wedgesprovide this capability. First, movement of the wedges (as a pair) in adirection orthogonal to the optical axis, or parallel with the opticalaxis, will not cause a change of focus or displacement of the image. Itis only the relative motion of one wedge with respect to another, whichcauses an effective change in the glass thickness placed in the opticalpath. The ratio of glass thickness variation to actual wedge movement isproportional to the wedge movement times the sine of the wedge angle.Also, the variation in path length is proportional to the effectiveglass thickness multiplied by the difference of the index of refractionof the glass and the external medium (air). The mechanical and opticaladvantage (between wedge motion and optical path length change) reducesthe sensitivity of mechanical error. With a wedge angle of about 6°, andan index of refraction of about 1.5, this mechanical advantage is 36to 1. Thus, a 0.0001 inch change in the optical path length (through thewedges) is achieved by a 0.0036 inch relative motion of two wedges. Thisis a tolerance which can be easily maintained and controlled.

Another aspect of the optical system which provides improvement is theuse of a projection lens 14 which is telecentric. The characteristic ofthe telecentric lens which is important here is that the principle raysat the microfiche side of the lens are all parallel and normal to thefiche's surface. This characteristic allows slight variations in thefocus which may be within the depth of focus, but would cause variationsof magnification with other types of lenses. Since it is intended thatthe automatic focus system be operated during the print cycle, theautofocus system will be continually operating, and although maintainingfocus within the depth of focus, may actually vary slightly causingminor but noticeable changes in the magnification if a non-telecentriclens were used. A second characteristic of a telecentric lens is that itis less sensitive to color aberrations caused by the various thicknessesof the glass path. This allows the use of the wedges for focus control.A third characteristic of the telecentric lens which makes it moresuitable for use with the type of focus described, is that the focusprobe can be placed off axis in the optical system without vignetting onthe return path. With a telecentric lens, the principle ray incidentthereon is refracted in such a manner that it strikes the other planenormal to the surface. The reflected ray thus reenters the lens, andvignetting does not occur.

It should be noted that the focus detector system utilizes a singlelight source which is split into two optical paths of differing lengths.This provides for measurement of focus condition on either side of theplane of best focus. The light returning from the platen via the twooptical paths is recombined and imaged on a single detector. It is thisconcept, the use of both a single light source and a single detector,which makes the system insensitive to changes in light intensity andchanges in detector sensitivity.

As noted, the system is extremely insensitive to stray light. Theability of the system to discriminate between the light energy of theimage and stray light due to forward scatter results in a much improvedsensitivity, as the percentage of light passing through the portions 26due to defocus is actually only a small percentage of the total lightdue to the high level scattering in such system. It has been found thata slight change in focus will produce only a one percent or two percentchange in the total light passing through the grid portions 26. Thissmall change could not be reliably detected without the modulatortechnique as described. As previously stated, the change in light leveldue to focus conditions is distributed at the edges of the portions 26,and it is only this light that is being modulated. The frequencyselective amplification, and synchronous detection of the modulationsignal, provide a means to extract the desired signal from the noise.Tests indicated that this system has an ability to detect the positionof best focus with a precision which is an order of magnitude greaterthan depth of focus of the projection system.

Another aspect of the optical system which provides improvement is thatthe optical probe focus system light path follows through the opticalpath used for projection and therefore does not require manual focusadjustment when magnification changes are made.

I claim:
 1. A system for automatically focusing images having differentmagnifications onto a light sensitive medium comprising:first means forprojecting a light image of a wavelength to which the light sensitivemedium is responsive along a first optical path from a projection planeto said light sensitive medium, said first optical path includingmagnification control means for changing the magnification of said lightimage by changing the length of said first optical path, second meansfor automatically focusing said image on said light sensitive mediumregardless of changes in the magnification of said light image, saidsecond means includinga. focusing means in addition to saidmagnification control means situated in said first optical path forchanging the length of said first optical path to achieve focus, b.means for projecting a first light pattern of a wavelength to which saidlight sensitive medium is not responsive along a second optical pathfrom a first light pattern producing means to approximately saidprojection plane and back to said first light pattern producing means,c. means for projecting a second light pattern of a wavelength to whichsaid light sensitive medium is not responsive along a third optical pathfrom a second light pattern producing means to approximately saidprojection plane and back to said second light pattern producing means,d. said second and third optical paths each including a substantial partof said first optical path including said magnification control means,and said first and second light pattern producing means each havinglight transmissive portions, and e. servo means coupled to said focusingmeans and oriented to receive any portions of said first and secondlight patterns which have followed said second and third optical paths,respectively, and have passed through the light transmissive portions ofsaid first and second light producing means, respectively, for producingmovement of said focusing means in a direction to bring said light imageinto focus on said light sensitive medium.
 2. The system of claim 1wherein said first and second light producing means are equallydisplaced from a point which is the same distance from said projectionplane as said light sensitive medium is from said projection plane. 3.The system of claim 2 wherein said focusing means is a pair of opticalwedges with one of said wedges coupled to said servo means and moving toachieve focus of said image.
 4. The system of claim 3 wherein said firstoptical path includes a telecentric projection lens.
 5. The system ofclaim 3 wherein said magnification control means is a pair of trombonemirrors.
 6. The system of claim 2 wherein means are provided adjacent tosaid first and second light pattern producing means for modulating saidportions of said first and second light patterns.
 7. The system of claim6 wherein said servo means includes means for combining said portions ofsaid first and second light patterns into an electrical signal, andmeans for demodulating said electrical signal to determine the directionof movement of said focusing means to achieve focusing of said opticalimage on said light sensitive medium.
 8. The system of claim 6 whereinsaid servo means includes means for combining said modulated portions ofsaid first and second light patterns to provide a proportional signal,means for amplifying said proportional signal at the modulationfrequency, means for synchronously demodulating said amplifiedproportional signal to provide a signal which indicates the direction ofmotion that said focusing means must move to achieve focus of saidimage, and drive means coupled to said focusing means and saiddemodulating means for moving at least a component of said focusingmeans to achieve focus of said image.
 9. The system of claim 8 furtherincluding additional means to determine whether said proportional signalis within the lock-in range of said servo means, said additional meansincluding a peak detector / comparitor circuit coupled to receive saidproportional signal and operative when the proportional signal is withinthe lock-in range of said servo system to permit activation of saiddrive means.